1. Field of the Invention
This invention relates to photodetectors and, more particularly, to using ion implantation to form Schottky barrier photodiodes.
2. Background of the Invention
It is known to form Schottky barrier diodes by evaporation or sputtering of a metal such as platinum, palladium, irridium, etc. onto a clean silicon wafer surface. The metal is then alloyed with the silicon at about 400.degree. C. to about 800.degree. C. to form the metal silicide, PtSi, Pd.sub.2 Si, IrSi, respectively. In using such a technique to form a diode, a significant problem is the preparation of the surface of the silicon wafer. The noble metals typically used, such as the above recited platinum, are not very reactive. If there is any oxide or other impurity on the surface of the silicon it will prevent the metal from forming a silicide and alloying. Creating a good vacuum system, capable of vacuums on the order of 10.sup.-9 torr, to prevent oxide formation at the surface of the silicon wafer is extremely difficult.
U.S. Pat. No. 4,499,483 to Yamazaki et al discloses a silicon photodiode having an n-type conductivty control layer 22 extending a distance into a silicon body 12. The n-type control layer is preferably formed by ion implantation of arsenic (As) or antimony (Sb). Thus, Yamazaki et al describe using ion implantation in the formation of silicon photodiodes sensitive to visible light. There is no teaching or suggestion how to form photodiodes sensitive to infrared radiation. A structure suitable for detecting visible radiation typically has different structural features and dimensions (e.g. thicker layers) than a structure suitable for detecting infrared radiation. Thus fabrication techniques to make one type of photodiodes would not be expected to transfer to the making of another type of diode.
U.S. Pat. No. 4,242,149 issued to King et al discloses a method of making a photodetector using ion implantation of a reactant dopant into a semiconductor substrate. In the embodiment shown in FIG. 1, a semiconductor substrate 10, such as cadmium telluride, is bombarded through a mask 11 with mercury ions. A laser is then used to heat the composite metal, allowing the mercury to combine with the cadmium telluride to form a mercury cadmium telluride photodetector. The photodetectors described are made with ion implantation only in composite material such as III-V and II-VI compounds. The techniques taught make no mention of class IV elements such as silicon and germanium. The photodetectors created are not of the Schottky barrier diode type.
U.S. Pat. No. 3,757,123 issued to Archer et al teaches metal deposition. The approach described has several drawbacks to commercial production. As mentioned above, the noble metals necessary for diode formation are very nonreactive. Any impurities on the silicon surface prior to deposition, such as silicon dioxide, prevent formation of a good quality Schottky barrier diode. This necessitates the use of very high vacuum deposition equipment which is expensive and has low throughput and poor repeatability.
U.S. Pat. No. 4,496,964 issued to Tsubouchi et al teaches a transistor and does not teach fabrication of a photodiode. As with some of the previously mentioned approaches, deposition is taught rather than implantation to form the silicide.
U.S. Pat. No. 3,971,057 issued to Connors et al teaches forming a Schottky diode by deposition. There is no mention of the thin layer required for infrared radiation detection or the use of such detectors for infrared detection.
U.S. Pat. No. 4,467,340 issued to Rode et al discusses a technique for interconnecting multiple photodetectors to suitable output circuitry in the form of a charge coupled device multiplexer. There is no description of any technique for forming a Schottky barrier photodiode array.
The above described deposition techniques for platinum as used for fabricating Schottky barrier photodiodes are often slow, expensive and nonrepeatable. This invention solves some of these aforementioned difficulties.